Description of the Prior Art
The Segregation Process
Polymer (and additive) segregation during melt flow of mixtures is a phenomenon which is generally considered a serious problem in the polymer industry because homogeneous mixtures are associated with good performance properties of the finished product. When a mixture of two immiscible polymers are flowing together in a system, the polymer with the lower viscosity will migrate to the region of higher stress. Although a variety of variables can affect the migration, the effect is often attributed to the "laziness of nature" or the "principle of energy minimization". Shear stress is the product of viscosity and shear rate. In fluid flow through a tube for example, the high shear rate area is adjacent to the wall of the tube and the low shear rate area is at the centre of the tube so that in this case, the low-viscosity component migrates towards the wall. The rate of this process depends upon the rate of change of stress across the tube.
The term additives in this document refers to: species previously added to improve performance of a finished product and no longer desirable in the case of recycling of the spent product; species previously inadvertently introduced into the polymers; species which appeared as a result of the manufacturing process (e.g. catalyst particles or microgel); species which have been added so that they can purposefully be located in a particular part of the polymer (e.g. a core of carbon particles may be desired). Additives can be soluble or insoluble in the polymer mixture. Additives of many types can segregate during flow. "Blooming", "bleeding" and "migration" are terms often used to describe the undesired segregation of plasticizers, dyes, etc. in polymer. The term polymer mixture in this document refers to any combination of polymers together or polymers and additives together.
Migration of particulate additives towards the centre of a tube during flow of particle in liquid suspensions through a tube can occur. One explanation for this effect is that any particle in the stream will have a slightly higher velocity on one side of it (the side closer to the axis of the tube) than on the other side. This causes a difference in pressure ("the Magnus effect") which pushes the particle towards the centre of the flow stream. Particle shape, particle size, particle deformability, particle concentration and the non-newtonian, viscoelastic nature of polymer melts provide additional variables. The effect causes difficulties in attempts to measure the viscosity of particle suspensions. Also, migration of filler particles during injection molding has caused important non-uniformities in molded parts.
There are a few areas where segregation is desired and used. The addition of water to heavy viscous crude oil in long pipelines has long been known to greatly reduce the pressure drop and hence the pumping cost. The water has much lower viscosity than the crude oil and therefore migrates to the wall of the pipe where it functions as a lubricant. Additives such as lubricants, slip agents and mold release agents function because they migrate to the surface of the polymer during processing.
Extrusion of polymer blends to form laminar structures without co-extrusion is receiving increased attention. In the publications H. Garmabi and M. R. Kamal, "The Development of Laminar Morphology In Extruded Polyethylene/Polyamide Blends", Proceedings Annual Technical Meeting Of Plastics Engineers (ANTEC'95), Boston, Mass., May 7-11, 1995; and G. W. Lohfink and M. R. Kamal, "Morphology And Permeability In Extruded Polypropylene/Ethylene Vinyl-Alcohol Copolymer Blends", Polym. Eng. Sci., 33, 1404-1420 (1993) showed that such structures were formed in the converging portion of the die unit attached to the extruder. They emphasize the need for equal viscosities of the polymer components in order to obtain the desired structures. In contrast to their approach, in the process to be described herein, differences in viscosity are desirable to segregate polymers because they are used to obtain desired structures through migration of the dispersed phase.
Since with these few exceptions, segregation is considered undesirable, the overwhelming bulk of prior art is directed at improved mixing of polymers and additives. This invention views segregation as desirable and provides a process for purposefully inducing segregation of polymers and additives.
Generation of Desirable Separate Product Streams from a Mixture of Polymers
Polymers, especially synthetic plastics, can contain undesired components, or higher concentrations of normally desirable components, that impair performance properties. These components are typically other polymers and additives (e.g. pigments and fillers). The situation is particularly acute for recycled plastics because they often originate from diverse sources. In recycled plastics the components of most interest are the types of polymers present. Thus, in conventional processing of such plastics, the waste is sorted manually according to the type of plastic. This is followed by washing, grinding, extrusion and pelletizing separately for each type.
The field of waste recycling is an area of considerable social and economic concern. The economic viability of the industry depends upon development of effective and efficient sorting methods. Sophisticated methods currently under study directed at solving this problem include the use of near infrared spectroscopy with neural network software to speed identification to enable rapid (possibly robotic) sorting. Triboelectric separation of comminuted waste, sink-float procedures based upon density differences, use of powder hydrocyclones, solvent techniques based upon the differing solubility of polymers in different solvents, and the use of chemical markers for identification are all methods presently being investigated in this area.
A major drawback in plastic waste recycling is that at present none of these methods provide satisfactory segregation. The most common method, manual sorting, is highly manpower intensive and expensive. Also, separating polymer waste composed of intimately mixed blends of components or of laminated components is not possible with current methods, except perhaps those that are solvent based. For those, solvent recovery and dissolution times are issues. Therefore, it would be very advantageous to provide a simple and economical method for segregating various components of polymer products.
Formation of Desirable Polymer and Polymer-Additive Structures
Polymer products often demand specific structures. In the packaging industry, for example, multiple layer structures are common as they are required to provide gas barriers and the like. These layered structures are traditionally coextruded using several extruders and associated special dies. In recent years there has been a recognition that extrusion of polymer blends can sometimes form such structures. For example, U.S. Pat. No. 4,410,482 issued to Subramanian on Oct. 18, 1983 discloses that extrusion of a blend of polyolefin, condensation polymer and compatibilizer can be accomplished so as to elongate the dispersed polymer phase to form a multi-layer structure. It is emphasized that the dispersed phase should not have too large a particle size. More recently, U.S. Pat. No. 5,1888,784 issued to Kamal et al. teaches dispersed "bodies" of ethyl vinyl alcohol dispersed in polypropylene are elongated using a die unit to obtain a laminar structure including overlapping layers of the dispersed polymer in the matrix. In that case different screw designs are used to affect the dispersion and the die unit is used to lengthen the dispersed bodies in different directions. All of this prior art emphasizes the presence of a dispersed phase and the stretching of the shape of the dispersed phase in a die. As mentioned above, most recently Garmabi and Kamal (1995) have emphasized the importance of equal viscosities in the two polymer phases so that the dispersed bodies are more deformed in the die so that larger areas are obtained in the laminar layers.
In addition to layered structures of various polymers, structures incorporating various additives as layers can also be important. One notable application is in the formation of electrically conductive polymer structures. Polymers with resistivities &lt;10.sup.6 ohm-cm are considered to be conductive polymers. Conductivity of a polymer can be the result of (a) conjugated bond structures which when activated provide mobile electron clouds or (b) the presence of electrically and/or thermally conductive additives. Examples for such additives include, conducting carbon black (CB), metal powders (Fe, Al), metal fibers (Al, SiC), metal coated glass fibers and hollow glass spheres. These additives are mixed with the polymers during processing in sufficient concentrations to form continuous conducting pathways in the polymer matrix. The matrix polymers employed are decided mainly by the application. Polymers often include non polar elastomers such as polyisobutylene, polyisoprene and conventional engineering plastics such as polycarbonate, ABS etc. and blends thereof. Application of such filled conductive polymers include, electromagnetic interference shielding (EMI), radar frequency jamming, antistatic applications and circuit protection in microelectronics.
In most of these applications, it is not necessary to have the whole matrix conductive, but one conductive layer will be sufficient serve the purpose. By suitably selecting a bi- (or multi) component polymer system such that conductive particles will preferentially migrate to one of the polymer phases (say the core as in an injection molded article) and the other phase forming an insulating shell around the core, could be constructed. Such in situ core-shell conductive-insulating structures have significant commercial potential including the production of electronic items. This technology is all the more desired as it may help retain the same amount of surface finish as without fillers, by reducing the material cost, by obviating multistage processing.
It would therefore be very advantageous to provide a method of producing polymer based layered products which does not require coextrusion of the various constituents.